The present invention relates to an information processing system, and more particularly to an information processing system in which logout is possible for a storage control unit which controls a main storage in response to a main storage access request from an I/O processing unit and/or a basic processing unit, and to a logout process therefor.
A modern information processing system, particularly a large scale information processing system typically comprises an organic combination of one or more main storages, storage control units, basic processing units, I/O processing units and maintenance control units. When an error occurs in any one of these units, it is necessary to output internal status information (error information) at the time of the occurrence of the error of the unit, analyze the status information and rapidly repair the fault unit where the error has occurred.
Since the main storage mainly consists of an assembly of memory elements, its reading and writing operations are controlled by the storage control unit. Thus, the main storage includes only a small number of logic circuits and not any error detector. The storage control unit is provided with an error detector for the main storage. When an error is detected, the storage control unit delivers its internal information for use in correcting the error. Meanwhile, when an error occurs in the I/O processing unit or in the basic processing unit, the fault unit in error is temporarily self-frozen at the time of the occurrence of the error so that the internal information at that moment is stored in the main storage via the storage control unit in preparation for the subsequent output of the information. In this manner, the error information for use in correcting the error is made available.
The error detection and correction discussed above are described in the chapter "Machine-Check Handling" of a manual entitled "IBM System/370 Principles of Operations" GA22-7000-3 published by International Business Machines Corporation in 1973.
However, when an error occurs in the storage control unit, it has been difficult to obtain error information in the same manner as was done when an error occured in the other units. Since the storage control unit always sequentially receives storage access requests from a plurality of processing units, if an error occurs in the storage control unit and the unit is temporarily frozen to store its internal information in the main storage, the basic processing unit and the I/O processing unit will detect a time-out for the storage access request and these units will stop operation. As a result, the entire system will stop its operation. It is a function of the maintenance control unit to read out the internal information of a fault unit and store it in the main storage (which function is called a logout). Because storing of information in the main storage is done via the storage control unit, it is apparent that the logout for the storage control unit will be impossible if the storage control unit is frozen.